The present invention relates to an improved cutter pump subassembly for use in a centrifugal cutter pump adapted for cutting and pumping a liquid containing entrained solids, such as a slurry. Pumps of this kind are used to transport fiber suspensions and various sludges such as sewage. Particularly when pumping sewage, this type of pump must be capable of cutting not only the softer solid particles carried in the liquid but also other impurities present such as paper, cloth products, and plastics.
Previous pumps developed to pump liquids containing entrained solids suffer the disadvantages of being unduly complex, thereby being difficult to construct and expensive to maintain. These previous pumps are also tend to provide a diminished head capacity, i.e., pressure differential from suction to discharge, because the chopping action restricts the flow of the liquid.
Another problems associated with previous centrifugal cutter pumps is cavitation. Cavitation occurs when the cutting action of the pump so restricts the flow of liquid through the pump so as to cause an abnormally low pressure to exist within the pump itself. When the pressure in the pump becomes too low, vapor forms in the pump housing. This vapor formation, or cavitation, inhibits the hydro-mechanical efficiency of the pump. Moreover, instantaneous collapse of the vapor bubbles causes severe impingement on the moving and stationary parts of the pump causing excessive wear on the pump chamber and impeller. Furthermore, due to the nonuniform forces encountered thereby, severe vibration is induced which causes damage to the shaft, seals, bearings, etc.
Many pumps in the prior art offer complex cutting, shredding, or chopping apparatus. One problem associated with these pumps is that the cutting action is not carried out in line with the natural flow of the liquid passing through the pump. Thus, the liquid flow is restricted. For example, in U.S. Pat. No. 4,145,008, issued to Wolferd on Mar. 20, 1979, a pump is shown in which the chopping action of the pump is provided by a blade spinning perpendicularly to the direction of fluid flow. This blade mechanism blocks the suction port of the pump, thereby restricting the fluid flow. Additionally, when the chopping action is not in line with the natural flow of the liquid, clogging can become a problem.
Another problem associated with some prior art devices is that of vibration during operation. If the chopping or cutting action occurs asymmetrically, or off-center, it can interfere with the natural fluid flow through the pump. Thus, asymmetric forces can cause the pump to vibrate, further causing wear on the shaft, seals, bearings and blades of the pump.
Moreover, these prior art devices, because of their complex design, are relatively fragile and easily broken. When repair becomes necessary, it may be less expensive to replace the entire cutting mechanism rather than the individual blade or tooth.
The present invention provides a design of a centrifugal cutter pump subassembly that seeks to overcome the problems discussed, while at the same time providing a simple, easily constructed design that is readily adaptable to a variety of influent liquids.